V1 extremely microphonic

[jewishjay]

I'm back for more help with my 2204 build. On a previous episode you guys helped me with the filter section that "went poof." So, with that handled, the amp now plays and sounds great.

HOWEVER, the 1st preamp tube is microphonic. It's not really a problem at normal playing volumes, but as you crank the gain and volume to higher settings it begins to howl and feedback, even without a guitar connected.

I've swapped tubes, it's not a bad tube. I"ve replaced a bunch of the preamp signal wires with shielded wire (not pictured) and that hasn't helped at all. Open to all suggestions...

[SILVERGUN]

It's not really a problem at normal playing volumes, but as you crank the gain and volume to higher settings it begins to howl and feedback

I don't see a problem, sounds like what everyone wants from their Marshall!
 

[mresistor]

How do you know it's the first preamp tube and not the others?

[shooter]

howl and feedback

throw an AU in and evaluate, happy, re-bias to happy AU R's

[wdelaney72]

The only place you need shielded wire is input jack to v1a. For the 68K gird stopper going to v1a, make sure that resistor is directly soldered on the tube pin (not on the circuit board).


You can also run shielded wire from the gain pot to the v1binput (I do), but it's not as important as the first stage. That 68K grid stopper has to be on directly on the tube socket.

[bmccowan]

I don't see a problem, sounds like what everyone wants from their Marshall!

That's funny but also has a lot of truth to it. And so does Shooter's recommendation of a lower gain tube. The problem you state sounds like the typical issue with high gain amps. Its a fine line - go over that line and you get noise/howl/feedback. Getting the right combination of voltage dividers can be a chore in my experience. And BTW, IMO there is no rule that says that grid stoppers on the input must be soldered directly to the tube sockets. It might help - it might not. I hate rules; always have. Message sent from prison.

[jewishjay]

How do you know it's the first preamp tube and not the others?

chop sticking
V1 is so sensitive, it seems like the source of the problem, tapping the chassis, and the circuit board make noise too, but I think V1 is the microphone, everything else is just in proximity.

i've tried a 12at7 and atleast it doesnt howl but it's still microphonic at high gain

[jewishjay]

there is no rule that says that grid stoppers must be soldered directly to the tube sockets. I hate rules; always have.

i know, huh?
how many fender amps have them floating off the jacks? and are problem free.

but, i really appreciate input from many voices. everyone's ideas are welcome, and will be considered.

[Willabe]

how many fender amps have them floating off the jacks? and are problem free.

Fender amps are not hi-gain amps, that's how they get away with not mounting them directly on the tube socket. 

Well known fact, grid stoppers work best attached directly to the tube socket grid pin, with as little of the resistors lead as possible used.

Not a 'rule' just how it works.

From Aiken Amplifiers;

https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

[sluckey]

The real reason for 68K resistors on the input jacks in fender amps predates Aiken's papers. An it had nothing to do with parasitic oscillation or radio frequency suppression. It was all about actually sharing those multiple inputs with several players. Even a microphone. When those 68K resistors first began showing up on amps in the '50s, it was quite common to see two guitars and even a mic plugged into a single amp. Still common in the mid '60s when I was first dreaming about a band. I had a Sears 1484 that my gitar buddy and I plugged into channel one and a cheap mic was plugged into channel 2. Had one hole free! The 68K resistors primary purpose was to provide isolation between the different instruments. Without that isolation, the VOL and TONE controls on my guitar would also affect my buddy's guitar. The 68K resistors prevented this interaction to a satisfactory degree, just as the 220K mixing resistors at the output of a two preamp Fender prevented interaction between the two channels.

At the time and for that purpose, the most logical and convenient place to put the isolation/mixing resistors was directly on the input jacks. Leo even figured out a clever way to provide useful HI/LO jacks in the event that only one person was using the amp. BTW, HI/LO only works for one person. The moment two people plug in, HI/LO is out the window.

Fast forward 40 years and NOBODY shares an amp. Everyone has their own. This makes multiple input jacks mostly useless. Now we have a radio station in every town, and high gain is popular. RF suppression and parasitic oscillation are more common. So, now the emphasis turns to grid stoppers and everything Aiken said is very important. And the most logical place to put them is directly on the grid pin.

[bmccowan]

it was quite common to see two guitars and even a mic plugged into a single amp

Reminds me of my 8th grade band, 1966. My Silvertone Dano, my bandmates Kent hollow body (he traded an early 60s Strat for it!) and the singer's mic plugged into a 1484. We were so bad. And this was before bad meant good.

[wdelaney72]

The real reason for 68K resistors on the input jacks in fender amps predates Aiken's papers. An it had nothing to do with parasitic oscillation or radio frequency suppression. It was all about actually sharing those multiple inputs with several players. Even a microphone. When those 68K resistors first began showing up on amps in the '50s, it was quite common to see two guitars and even a mic plugged into a single amp. Still common in the mid '60s when I was first dreaming about a band. I had a Sears 1484 that my gitar buddy and I plugged into channel one and a cheap mic was plugged into channel 2. Had one hole free! The 68K resistors primary purpose was to provide isolation between the different instruments. Without that isolation, the VOL and TONE controls on my guitar would also affect my buddy's guitar. The 68K resistors prevented this interaction to a satisfactory degree, just as the 220K mixing resistors at the output of a two preamp Fender prevented interaction between the two channels.

At the time and for that purpose, the most logical and convenient place to put the isolation/mixing resistors was directly on the input jacks. Leo even figured out a clever way to provide useful HI/LO jacks in the event that only one person was using the amp. BTW, HI/LO only works for one person. The moment two people plug in, HI/LO is out the window.

Fast forward 40 years and NOBODY shares an amp. Everyone has their own. This makes multiple input jacks mostly useless. Now we have a radio station in every town, and high gain is popular. RF suppression and parasitic oscillation are more common. So, now the emphasis turns to grid stoppers and everything Aiken said is very important. And the most logical place to put them is directly on the grid pin.

That's a great point....and interesting.


Most of the amps I've worked on are Marshalls....yes Marshall was based on the early Bassman, but by the time the 2203/2204 came out, shared instruments were long done. Marshall mounted all their 2203/2204 directly to the socket, as does the Soldano. The Marshall Jubiilee....directly to the socket. Any amp that has the cascading gain stages needs to be careful of that first stage. So yes, makes sense that Fenders did not, but they weren't trying to do higher gain like the others at this point.


Of course, we can now find the exception....Mesa MKIIC+......no grid stopper at all on V1  . Still....consensus on most is that for cascading higher gain amps, this is a must on the 1st stage.

[sluckey]

Marshall mounted all their 2203/2204 directly to the socket

Not so. The 68K grid stopper for the 2204 is mounted on the board (third resistor from left). This is Mark Huss's JCM-800 2204...

[scissorspaperockstar]

HOWEVER, the 1st preamp tube is microphonic....
I've swapped tubes, it's not a bad tube. I"ve replaced a bunch of the preamp signal wires with shielded wire (not pictured) and that hasn't helped at all. Open to all suggestions...

Thankyou for my acceptance into the forum. I have a JCM 800 50W bass head I bought in London in 1984 and carried back to Australia in my cabin luggage  . The layout is coincidently the same as the one in Slackey's picture here, and like JJ's build issue the 1st preamp tube is microphonic (pencil tap) but concluding after swapping round the preamp valves, NOT to be the valve but the socket. The amp hums at higher volume with various different tones cranking the other pots.
I first cleaned the socket with alcohol and contact cleaner on a dental stick to no change, but then after reflowing the socket connectors it went nice and quiet. I switched it off and left the project a week or two but then coming back today and powering on the tube has gone MF again, and yes I swapped the tubes again and the issue stayed with the socket. So I completely un-soldered then re-soldered the socket wires, just to be careful and no improvement.

I would not hesitate to replace the socket but the 1986 amp is lightly used with all original tubes and replacement power caps recently as one started leaking years back. I cant understand how the socket could have failed having no vioisible cracks or corrosion and besides how could such a basic connection device go bad?

Any suggestions or experience welcome.

Footnote - the socket in question is actually the one on the far left of Slackey's pic, which just happens to be only 2 inches away from the former leaking power cap, and I noticed on first inspection of the socket a slight amount of residue on its topside around the centre hole. 

[Jennings]

I concur…obviously check the valve and lead dress etc, but I have found dirty or poorly grounded valve sockets/connections in key stages to be naughty little culprits

[Platefire]

Ya'll got me wondering if I installed my V1a grid stopper resistor on the board or on the socket pin on my Marshall Clone?   Looked at the pixs and it was to the socket pin

[tubeswell]

HOWEVER, the 1st preamp tube is microphonic....
I've swapped tubes, it's not a bad tube. I"ve replaced a bunch of the preamp signal wires with shielded wire (not pictured) and that hasn't helped at all. Open to all suggestions...

Thankyou for my acceptance into the forum. I have a JCM 800 50W bass head I bought in London in 1984 and carried back to Australia in my cabin luggage  . The layout is coincidently the same as the one in Slackey's picture here, and like JJ's build issue the 1st preamp tube is microphonic (pencil tap) but concluding after swapping round the preamp valves, NOT to be the valve but the socket. The amp hums at higher volume with various different tones cranking the other pots.
I first cleaned the socket with alcohol and contact cleaner on a dental stick to no change, but then after reflowing the socket connectors it went nice and quiet. I switched it off and left the project a week or two but then coming back today and powering on the tube has gone MF again, and yes I swapped the tubes again and the issue stayed with the socket. So I completely un-soldered then re-soldered the socket wires, just to be careful and no improvement.

I would not hesitate to replace the socket but the 1986 amp is lightly used with all original tubes and replacement power caps recently as one started leaking years back. I cant understand how the socket could have failed having no vioisible cracks or corrosion and besides how could such a basic connection device go bad?

Any suggestions or experience welcome.

Footnote - the socket in question is actually the one on the far left of Slackey's pic, which just happens to be only 2 inches away from the former leaking power cap, and I noticed on first inspection of the socket a slight amount of residue on its topside around the centre hole.

Better to start a new thread than reply to a thread that’s several years old.


Have you got a hi-res gutshot of the amp you can post? Are there any bad cliff jacks in the amp where the tip switch isn’t closing properly?